Radio-frequency rotating joint



March 29, 1949- D. w. PETERSON I RA DIO FREQUENCY ROTATING JOINT Filed July 14, 194:5

flamilrzwii i l llllllll ll Gttorneg Patented Mar. 29, 1949 2,465,922 RADIO-FREQUENCY ROTATING JOINT Donald W. Peterson, P

Radio Corporation of of Delaware Application July rinceton, N. J., assignor to America, a corporation 14, 1943, Serial No. 494,617

4 Claims. (Cl. 178-44) This invention relates to radio frequency distribution systems and more particularly to improvements in rotatable couplings or joints for concentric transmission lines used for transferring radio frequency energy between a rotatable antenna and a fixed transmitter or receiver.

In the operation of directional radio devices it is frequently necessary or desirable to rotate the antenna continuously. This necessitates a rotatable coupling in the transmission circuit between the antenna and the radio apparatus. One prior art type of rotatable coupling comprises a pair of concentric line sections, with outer conductors and inner conductors respectively coupled through slidable spring fingers, thus providing conductive coupling between the two transmission line sections. One disadvantage of such an arrangement is that the contacts tend to become erratic through wear caused by continuous use with the result that arcing may cause them to destroy themselves. Also small particles of material become dislodged and fall into the bearings and the space between the inner and outer line conductors. An accumulation of such particles is likely to cause destructive arcing across the line, particularly where large peak power is being transmitted.

Some prior art devices avoid the above mentioned difliculties but introduce undesirable frequency characteristics, as described more fully hereinafter.

It is the principal object of this invention to provide an improved rotatable coupling for radio frequency transmission lines.

Another object is to provide an improved coupling of the described type embodying no movable or slidable contacts.

A further object is to provide a coupling which will operate efficiently over a broad band of radio frequencies. These and other objects will become apparent to those skilled in the art upon consideration of the following description, with reference to the accompanying drawing, of which Figure l is a schematic diagram of one type of prior art coupling,

Figure 2 is a schematic diagram of a rotatable joint according to the present invention, and

Figure 3 is a longitudinal section of a rotatable joint embodying the principles illustrated by Fig. 2.

Referring to Fig. 1, a known type of rotatable joint involving no slidable contacts is shown. Two concentric line sections l and 3 are arranged so that their inner conductors and outer conductors respectively overlap by a distance d.

The diameter of the outer conductor 5 of the line I is slightly less than that of the outer conductor 1 of the line 3, so that the conductors 5 and 1 are closely adjacent to each other but do not touch throughout the overlapping portion. The inner conductor 9 of the line I is hollow at least through the overlapping portion, having an inner diameter slightly greater than the diameter of the inner conductor ll of the line 3.

The operation of the above described structure is as follows:

The overlap d is adjusted to approximately wavelength at the frequency at which the equipment is to operate. The overlapping outer conductors 5 and 1 constitute a low impedance concentric line, open-circuited at its outer end l3. Owing to the well known impedance inversion characteristics of /4 wave lines, the open circuit at the point 13 is reflected as a short circuit at the point l5, thus effectively connecting together the outer conductors 5 and l at this point. Similarly the inner conductors 9 and H act as a /4. wave line open circuited at the point i1 and present a short circuit at the point 19. Thus at the frequency for which d is equal to a wavelength, the inner conductors 9 and II are effectively connected together at the point 19. If the clearances between the overlapping portions of the conductors of the lines I and 3 are relatively small, the characteristic impedances of the wave open circuited lines are relatively low, and the impedance presented at the points l5 and I9 are substantially zero over a fairly wide frequency range. A typical joint constructed according to Fig. 1 is capable of transferring energy to a matched line over a range of 300 to 500 megacycles with a standing wave ratio of 85 percent or better over a range of 1.67 to 1.

According to the present invention a usable frequency range of the order of three to one is obtained by spacing the series reactances of the low impedance sections formed by the overlapping conductors so that reactance cancellation occurs at two frequencies respectively above and below the midband frequency. Referring to Fig. 2, the lines i and 3 are arranged with their conductors in overlapping relationship as in the system of Fig. 1. The outer conductors 5 and 1 and the inner conductors 9 and H overlap by dis tances d, where d is approximately wavelength near the center of the frequency band over which the system is to operate. The overlapping portion of the outer conductors 5 and l is separated from the overlapping portion of the inner conductors 9 and II by a distance which U is the distance from point IE to point l9 and is somewhat greater than a wavelength at the midband frequency.

The operation of the above described coupling at the midband frequency is substantially identical to that of the structure of Fig. 1. The open circuits at points I3 and H are reflected as short circuits at points l5 and l 9, effectively connecting the outer conductors 5 and 1 and the inner conductors 9 and II respectively. At frequencies below the midband frequency, the overlapping conductors present capacitive reactances at points 55 and IS. The line section of length 8 between the points l5 and I9 inverts the capacitive reactance at the point l5 to inductive reactance at the point [9, tending to cancel the capacitive reactance presented at the point I9 by the line comprised of conductors 9 and I1. At some fre- 85 percent or better, or band width over which the arrangement of Fig. 1 will provide equivalent operation.

Fig. 3 shows one form tion of a joint of the type schematically illustrat- 7 ed in Fig. 2. A tubular flanged member 33 and a disk 34 of insulating material, and is provided at its other end with a threaded fitting 35 for engagement with the outer at the point 63 to a tubular extends over the rod 4H and by insulators ll and 49.

conductor 45, which is supported thereon The conductors 37 and of coaxial transmission lines. line conected to the members 29 In operation, the and 5! is rotated 3| and the conductors 4| and 45 and the spacings between the overlapping portions are proportioned as described above in connection with Fig. 2.

Thus the invention has been described as an improved rotatable joint for radio frequency transmission lines providing efficient transfer of energy over a band of frequencies without the use of sliding metallic contacts. The inner and outer conductors of the sections to be joined overlap by substantially a A; wave length in the middle of the band to be transmitted, providing a perfect match at that frequency. The overlapping portion of the outer conductors is longituditively above and below the midband frequency.

I claim as my invention: 1. A joint for connecting a rotatable coaxial transmission line to a stationary transmission ductors, the overlapping portion of said second pair of conductive members being by a distance of approximately one-quarter wavelength at the center of the band of frequencies over which the device is to operate spaced longitudinally along the axis of said lines from the overlapping portion of said first pair of conductive members by a distance somewhat greater than the length of said overlapping portions and substantially less than twice said length.

2 A device of the class described, including a pair of concentric radially spaced, overlapping outer conductors insulated from each other, a pair of concentric radially spaced overlapping inner conductors insulated from each other and from said outer conductors, the lengths of the overlapping portions of said conductors being such as to present substantially zero reactances to currents of a predetermined frequency, and capacitive and inductive reactances to currents of frequencies respectively below and above said predetermined frequency, said overlapping portions being spaced longitudinally of said device to provide impedance inversion and substantial cancellation of said reactances at two predetermined frequencies respectively above and below said predetermined frequency.

3. A device of the in radially spaced overlapping relationship and insulated from each other, a second pair of concentric radially spaced overlapping conductors disposed within said first pair and coaxi ally therewith and insulated from each other and from said the overlapping portion of said first pair by a distance substantially less than one quarter wave length at the center frefrequencies over which the device is to operate, th length of each of said overlapping portions being approximately one quarter wave length at said center frequency.

4. A device of the class described, comprising a pair of concentric tubular conductors disposed in radially spaced overlapping relationship and insulated from each other, a second pair of concentric radially spaced overlapping conductors disposed Within said first pair and coaxially therewith and insulated from each other and from said first pair of conductors, with the overlapping portion of said second pair of conductors spaced longitudinally from the overlapping portion of said first pair, the lengths of said overlapping portions being approximately one-quarter wave length at the center of the band of frequencies over which the device is to operate, corresponding points on said overlapping portions being spaced longitudinally by a distance somewhat greater than the length of said overlapping portions and substantially less than twice said length.

DONALD W. PETERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

